1. Field of the Invention
This invention relates to a semiconductor pressure sensor having a diaphragm formed by anisotropically etching a monocrystalline silicon substrate having a surface orientation of substantially (110) or (100).
2. Description of the Related Art
FIG. 1 shows a conventional semiconductor pressure sensor. This semiconductor pressure sensor has a recessed diaphragm 4a having a substantially square shape formed by anisotropically etching a monocrystalline silicon substrate 1a having a surface orientation of substantially (110). With this surface orientation, strain gauges for forming a bridge circuit are arranged on the periphery and in the center of the diaphragm 4a.
In FIG. 2, a non-diaphragm area 5 of the substrate 1a is joined with a base 3 made of, for example, Pyrex (trade mark) glass. The base 3 has a pressure introducing hole 31 through which pressure is introduced from the outside to a recessed face of the diaphragm 4.
Japanese Examined Patent Publication No. 60-13314 discloses another example of the semiconductor pressure sensor. This sensor has an octagonal diaphragm with sides extending parallel with two different crystal axes on the surface of a semiconductor substrate having a surface orientation of (100). In the sensor having this surface orientation, all strain gauges are usually arranged on the periphery of the diaphragm. This arrangement prevents a local concentration of stress and increases the maximum allowable applied pressure.
In the conventional semiconductor pressure sensor comprising the monocrystalline silicon substrate joined with the base, the diaphragm is subjected to thermal stress due to a difference in the thermal expansion coefficient between the substrate and the base. The thermal stress in the diaphragm fluctuates and causes the bridge circuit composed of the strain gauges to provide a thermal stress output. This output is an unwanted signal component (an offset voltage) mixed with a pressure signal. The offset voltage is usually nonlinear with respect to a temperature change and difficult to be compensated for in a simple electronic circuit, thereby deteriorating the accuracy of the pressure sensor. No effective measures have been proposed so far to solve this problem.
When fabricating semiconductor pressure sensors, it is also known to use a semiconductor substrate with respect to each of the two principal planes (110) and (100) and etch (preferably anisotropically) the substrate to form a diaphragm. This will be explained with reference to FIGS. 3 and 4.
In the figures, a silicon substrate 1a has a surface orientation of (110). Strain gauges 2a are symmetrically arranged to form a bridge circuit on a front principal plane of the silicon substrate 1a. With the center of symmetry of the strain gauges serving as an etching center, a back principal plane of the silicon substrate 1a is anisotropically etched into a truncated pyramid to form a thin diaphragm 4a.
In a direction orthogonal to the principal plane, the center "s" of an etching start surface S2 defined by a photo mask from where the anisotropic etching is carried out agrees with an intermediate point (gauge center) "g" of the strain gauges 2a.
The diaphragm 4a deflects due to a difference in pressure between the front and back faces thereof, and accordingly, the strain gauges change their resistances to provide a voltage signal from an output terminal of the bridge circuit.
A semiconductor integrated circuit, usually arranged as an external circuit includes bipolar transistors for amplifying the signal from the bridge circuit and compensating the signal for temperature. The external circuit may be arranged on the silicon substrate 1a to reduce the size of the semiconductor pressure sensor. To fabricate such bipolar transistors, an epitaxial layer must be formed on the silicon substrate 1a, and to reduce crystal defects of the epitaxial layer, the substrate 1a must have the off-angle. Namely, the silicon wafer must be sliced at a slight offset such that the principal plane of the substrate is inclined, i.e., has an off-angle, by several degrees with respect to planes (110) and (100).
When the substrate having the off-angle is anisotropically etched, a pair of slant faces (for example, slant faces 18 and 19 in FIGS. 3 and 4) extending from the sides of the diaphragm 4a are differently inclined with respect to the principal plane of the substrate by the amount of the off-angle. If the off-angle is 3 degrees in FIGS. 3 and 4, the slant face 18 has an angle of 32.3 degrees and the slant face 19 an angle of 38.3 degrees with respect to the principal plane (110).
As a result, the center s of the etching start surface, that is, the surface of the substrate before etching to form the diaphragm is started, that agrees with the gauge center g in the direction orthogonal to the principal plane may not agree with the center of the diaphragm 4a, i.e, the center "b" of an etching end surface S1, that is, the surface of the substrate appearing after it is etched to form the diaphragm. Due to this, a pair of the strain gauges arranged, in particular, in the center of the diaphragm 4a on opposite sides of the bridge circuit may achieve unequal strain-resistance characteristics to adversely influence the output characteristics of the bridge circuit.